The surface level of various materials can be measured in open or closed containers by indirect distance-measuring methods utilizing, for example, radiometric, optical, acoustic and electric techniques.
Such methods have been used to indicate or detect the surface levels of the flowable contents of bins or like containers, whether of solids (sometimes referred to as bulk materials) or liquids (sometimes also included within the description of bulk materials).
For the purposes of this description, the term "flowable material" may be used to indicate solid bulk materials and liquids stored in bins or like containers.
It is also known to determine the surface level of the contents of bins or like containers by means of apparatus which indicate the degree of filling by purely mechanical means, e.g. with the aid of a diaphragm. Such indicating means is generally effective if the material in the bin consists of small particles which are uniformly shaped and are dry and freely flowable, or are liquids.
However, where the particles of the material differ substantially in size or in shape so that flow does not occur in a uniform manner and/or if the material tends to adhere to the walls of the container, difficulties are encountered in the use of conventional mechanical level detectors.